Tried something similar with Cubic, and it sure saves very much render time, although your technique is way faster. :) And apart from the technique, thats a kickass scene! Even as a greyscale only! Btw, heres another trick. Duplicate the existing layer, add a similar amount of gaussian blur like the one you have above. Then choose "lighten" around 35-50% on the blurred layer. This will give the image a soft glowy atmosphere.

Rudolf Herczog
Digital Artist
www.rochr.com